The invention is directed to a filter system for filtering fluids, particularly gases, particularly for filtering air streaming into the passenger cabin of a vehicle, comprising a filter element folded in a wave-shaped or zig-zag-shaped manner having bending lines and substantially plane filter element folding portions arranged therebetween, the filter element being adapted to be stretched in the direction of its successive folding portions and to be compressed in the opposite direction.
A pleated paper filter is known from French Patent No. 1,289,456, provided with longitudinal paper glued to the edge of the filter. The paper filter is maintained in the pleated zig-zag condition by a rigid peripheral framework in conjunction with longitudinal and transverse support wires. The transverse wires are attached to two parallel longitudinal-wires side edges of the filter. The transverse support wires engage the filter at the bending lines of each pleat fold maintaining the pleat structure and spacing.
Filter systems for filtering air streaming into the passenger cabin of a vehicle are known in a wide range of versions. Examples of such known filter devices are described in German Laid-open Application Nos. 25 57 444, 35 29 579, 35 34 318, 37 16 243 and 38 15 907, as well as in German Patent Specifications 21 38 412 and 34 39 255 and, finally, in German Utility Model 80 01 798. A common feature of all these filter systems resides in that the filter element is fixedly mounted in a filter frame so that exchange of the filter element is not possible without exchanging the filter frame along with it.
In known filter systems, the filter element is arranged in wave- or zig-zag-shaped folds for enlarging the filtering area. Since the filtering materials (for instance, paper or non-woven materials) have hardly any inherent stability of shape, there is either added an air-permeable reticular support structure to the filter element, or there are provided spacers for keeping the substantially plane folding portions of the filter element between successive bending lines of the filter element at mutual distances and thus maintaining said filter element folding portions in their V-shaped orientation relative to each other. The provision of spacers complicates the construction of the filter system.
The filter system known from EP 0 170 643 comprises a filter element adapted for insertion into a filter frame. The filter element is accommodated in the filter frame in a manner allowing exchange of the filter element. For providing stability of shape of the zig-zag-shaped filter element, also this known filter system includes special mutually engaging teeth on the two-part filter frame. By such teeth or spacers, not only the construction of the filter system but also the process of inserting a new filter element becomes more complicated.
It is the object of the invention to provide a filter system wherein-the filter element is automatically given stability of shape with increased stretching thereof.
For solving the above object, the invention provides a filter system of the initially mentioned type, wherein the filter element is provided with two tensioning members, which have a modulus such as to limit the stretching of the filter element and which also fix the filter element folding portions when the tensioning members are elongated under tension, and wherein the tensioning members at the two side edges of the filter element extending in stretching direction are tightly connected to the filter element.
In the filter system of the invention, the filter element is provided with a respective tensioning member on each of its two edges extending in stretching direction. This tensioning member is resistant to tensile forces and will not stretch to any noteworthy extent when subjected to tension; for instance, the tensioning member can be a flexible tensioning band resistant to tensile forces and being hardly stretchable. Typically, the tensioning member will be an inelastic, high modulus, flexible web. This tensioning band or tensioning member is fixedly connected to the edges of the filter element being wave-shaped in side view. The flexibility of the tensioning members allows compression of the filter element in a direction opposite to their stretching direction; then, those portions of the tensioning members which are not connected to the filter element will become folded or will collapse in wave shapes.
The tensioning members at the edges of the filter element limit the extent of the stretching of the filter element. When stretched up to their limit, the high modulus tensioning members are subjected to tension and are tightly strung. Because of the firm connection of the edges of the filter element to the tensioning members, no movement is possible for the individual filter element folding portions oriented in V-shaped manner to each other; this is because the filter element folding portions are held in position at their lateral ends by the tightened and rigid tensioning members. In the filter element of the filter system of the invention, additional spacers, protruding into the V-shaped intermediate spaces between adjacent filter element folding portions, are not required so that the construction of the filter frame receiving the filter element can be simplified, if desired.
Preferably, the filter element comprises a reticular support structure holding a filter material layer, preferably an electret filter material layer. The reticular support structure, being comparatively bending-resistant, is folded corresponding to the folding portions of the filter element and, on its lateral edges extending in the stretching direction, is connected to the tensioning members, e.g., by gluing or welding. The filter material layer does not necessarily need to be connected to the tensioning members, but preferably is connected thereto. The reticular support structure is many times more resistant to tearing than the filter material layer.
Preferably, the tensioning members are provided as tensioning bands whose width is equal to or substantially equal to the horizontal distance between two adjacent bending lines (folding or bending lines). Accordingly, the bending lines of the filter element on the upstream flow direction extend at the one longitudinal edge of the tensioning band while the downstream bending lines of the filter element are arranged at the opposite parallel side edge of the tensioning band.
For assembly (connection) of the tensioning members with the filter element, the filter element is first stretched to the desired extent so that adjacent filter element folding portions extend in a specific angle to each other. In this condition of the filter element, the tensioning bands or tensioning members are glued, attached by ultrasonic welding, or fastened in some other suitable manner on both longitudinal sides. During fastening, the tensioning bands or tensioning members are in the fully stretched condition. When later stretched, the filter element will not extend beyond the point where the filter element was fastened to the tensioning members. Thus, when the filter element has been stretched to the limit, adjacent filter element folding portions extend at an angle or in V-shaped orientation to each other.
Preferably, the substantially non-stretchable or high modulus tensioning members consist of a fibrous web material resistant to tensile forces, particularly a strengthened non-woven material of thermoplastic fibers. Upon insertion of the filter element in the filter frame, the tensioning members or tensioning bands abut two opposite inner faces of the filter frame. A fibrous or non-woven material inevitably has the effect that individual fiber protrude out of the surface of the tensioning band. These fibers abut on the inner faces of the filter frame and in this region exert a filtering function on any flow of the medium in this region.
By strengthening the non-woven material through treating and processing methods known for that purpose, such as heat treatment and/or calendering, it is also possible to utilize fiber materials for the tensioning members, which is advantageous because of the above mentioned additional filtering effect of the projecting fiber ends. Particularly, the tensioning members consists of thermoplastic (staple) fibers being multiply thermally bonded and randomly arranged.
The filter frame for insertion of the filter element therein is provided with a stretching means for stretching the filter element and for holding the filter element in the stretched condition thereof. This stretching means may be arranged on one filter frame portion only; however, it is also possible that each of the two ends in the longitudinal direction of the filter element has a stretching means assigned thereto. By means of said one or plurality of tensioning means, the filter element is stretched within the filter frame to such a degree that the tensioning members are subjected to a pulling force and are tightened. The stretching means can engage either the ends of the tensioning members or the outer filter element folding portions. Advantageously, the stretching means is connected to a locking element by which the part of the stretching means providing the stretching force is locked in the stretching position for stretching the filter element.
In an advantageous embodiment of the stretching means, the filter frame is provided with two holding members for holding the two outermost filter element folding portions in the stretch direction, said outermost filter element folding portions being adapted to be mounted onto said holding members from above while a complete, or at least partial, stretching of the filter element is effected. The holding members, being e.g., plate-like elements, are preferably articulated on two opposite portions of the filter frame, and then preferably on the rear inner edges of the respective filter frame portion when seen in the flow direction. For stretching, at least one of the holding members can be pivoted on a pivot point in a direction away from the other one and toward the filter frame. For securing the movable holding member against return movement due to mechanical stress of the stretched filter element, there is provided a locking hook for locking the holding member in its pivoted position against the inner face of the filter frame. This locking hook can also be provided for engagement above the filter element at one or both of the outer bending lines. Since the holding member engages the filter element folding portion from below and extends up to the bending line, the holding member will be held by the locking hook gripping the filter element.
The filter system of the invention is relatively inexpensive in manufacture. Manufacturing costs are decreased particularly because the filter element itself need only have a comparatively low stability of shape because the stability of shape, i.e., maintenance of the filter element folding portions in their angular positions to each other, is effected by the tensioning members. Filter elements with reticular support structures or the like, can be provided allowing the filter element to be less strong and stable without the danger of ballooning or collapsing of adjacent filter element folding portions in their V-shaped orientation.
In an advantageous embodiment of the invention, the filter element is provided with a reticular support structure having connected thereto an electret filter layer of a non-woven material, the layer of non-woven material preferably being arranged behind the reticular support structure when viewed in the flow direction, and the electret filter layer is freely expandable to the side facing away from the reticular support structure and the flow direction. Both the reticular support and the electret filter would be fastened to the tensioning means. The non-woven material of the electret layer is connected to the reticular support structure by gluing, welding or in a mechanical manner and therefore is (directly or indirectly) held on the reticular support structure. Thus, the medium flow passing through the filter element first penetrates the permeable reticular support structure and then the nonwoven material of the electret filter layer while widening this loose non-woven material layer; the resultant enlargement of volume leads to an increased storage capacity of the filter element and thus longer service life. Particularly, due to the enlarged distance of the individual fibers of the electret filter layer, more space is available around the fibers for the binding of particles through the electrical charge of the individual fibers. Since the electret filter layer is covered by the reticular support structure on the flow receiving side of the filter element, protruding fibers of the non-woven material are pressed down so that only very few of these fibers will extend through the reticular support structure. Because of the reduced number of fibers of the nonwoven material projecting into the V-shaped intermediate spaces, the turbulences generated in these V-shaped recesses will be weaker. In other words, for having to tolerate the same turbulences as in a filter element having the non-woven material arranged before the reticular support structure as seen in the flow direction, the wave shape of the filter element can be made denser. This means that the number of folds, i.e, the number of filter element folding portions per unit of length, can be increased. Thus, the folding portions can extend at still more acute angles to each other so that the filter area is enlarged. With the same air resistance and an enlarged filter surface, there is finally obtained an improvement of the air resistance by the arrangement of the non-woven material layer behind the reticular support structure.
Accordingly, the increased storage capacity in the filter element in the filter system of the invention is realized, on the one hand, in that the filter element layer can freely expand in the fluid flow and thus have its volume enlarged; and on the other hand, contributing to an enlarged storage capacity in that a constant air resistance allows an increase of the filtering area. Also, the arrangement of only one electret filter layer behind the reticular support structure improves the flow characteristics of the filter element.
In a further embodiment of the invention, the layer of nonwoven material on the side or surface thereof facing away from the reticular support structure is provided with a cover layer for keeping the non-woven material together. As mentioned above, the non-woven material layer can widen during the passage of the medium through the filter element. To keep the non-woven material of the filter material layer from "flying off" the fiber material is bound or held together also on its free side facing away from the reticular support structure by the cover layer.
This cover layer does not restrict the expansion of the non-woven material instead, when the filter element has fluid passing therethrough, the cover layer will move away from the reticular support structure to the extent to which the non-woven material is expanded. The cover layer merely fulfills the function of keeping the non-woven material together for preventing that individual fibers become detached from the electret filter layer or are torn out of it while the fluid flow is passing through the filter element.
Preferably, the cover layer consists of a strengthened non-woven material. The strengthening of the non-woven material can be performed by the usual treating and processing techniques known for that purpose. Particularly, the cover layer consists of thermoplastic (staple) fibers being multiply thermally bonded and randomly arranged. The cover layer has greater strength (against tearing) than the electret filter layer whose non-woven material even if strengthened in a certain manner, e.g., by being needle tacked -- are connected in a relatively loose manner as compared to the cover layer. However, this strengthened nonwoven material has no inherent stability, i.e., the zig-zag-shaped folds of the filter element are maintained by the correspondingly folded or formed reticular support structure and tensioning members.
Advantageously, both the reticular support structure and the non-woven material (of the filter material layer and, if provided, the cover layer) are the same thermoplastic material, preferably polypropylene. It is preferred that all of the components of the filter element and of the overall filter system consist of the same thermoplastic material, which is advantageous for recycling purposes polypropylene has been found to be particularly useful for the filter system.
The filter system of the invention is preferably placed into the to-be-filtered fluid flow in such a manner that the reticular support structure is the first layer of the filter element to come into contact with the fluid. The reticular support structure is then followed by the electret filter layer and -- if provided -- the cover layer. For maintaining the fluid flow, there is provided a suitable means, e.g., a blower or the like, for blowing the air through the filter element. The term "means" has a very wide meaning in the context of the invention; it is meant to comprises all devices and circumstances providing for a (fluid) flow passing through the filter element. Ultimately, such a means can also be the vehicle itself because the vehicle, provided with the filter system of the invention, generates an airflow while being driven, with the airflow streaming through the filter element of the filter system .